Most heat-curing silicone rubbers are based on high molecular weight silicone polymer gums. Gums, fillers, and additives are mixed in dough mixers or Banbury type mixers or mills to produce the heat curable formulation. Curing catalysts are added on water cooled rubber mills, to avoid premature heat cure, which can sometimes be used for the entire formulation in small-scale processes.
Silicone rubbers are commercially available as gums, filler-reinforced gums, dispersions, and uncatalyzed and ready-to-use catalyzed mixtures. The following types of gums are commercially available:
1) general purpose gums based on methyl and vinyl gums, PA0 2) high and low temperature gums based on phenyl, vinyl, and methyl gums, PA0 3) low compression set gums based on methyl and vinyl gums, PA0 4) low shrink gums, i.e. gums which have been devolatilized, and PA0 5) solvent resistant gums, based on fluorosilicone gums. PA0 1) hydrosilylation, PA0 2) free radical initiation, and PA0 3) high energy radiation initiation. PA0 1) in-situ filler treatment, PA0 2) post-reaction catalyst inhibition, and PA0 3) additives. PA0 (1) an alkenyl silicone gum or mixture of alkenyl silicone gums selected from the group of alkenyl silicone gums having the formula: EQU (M.sub.a M.sup.vi.sub.1-a)(D.sup.vi).sub.x (D).sub.y (M.sub.a M.sup.vi.sub.1-a) PA0 (2) a vinyl specific peroxide curing agent. PA0 (1) M.sup.vi D.sup.vi DM.sup.vi ; PA0 (2) M.sup.vi DM.sup.vi ; and PA0 (3) MD.sup.vi M. PA0 (a) from about 5 parts by weight to about 100 parts by weight of a vinyl on chain vinyl stopped gum having the formula: EQU M.sup.vi D.sup.vi.sub.x D.sub.y M.sup.vi PA0 (b) from about 0.2 parts by weight to about 95 parts by weight of a vinyl stopped gum having the following formula: EQU M.sup.vi D.sub.z M.sup.vi PA0 (c) from 0.2 parts by weight to about 75 parts by weight of a vinyl on chain gum having non-reactive end groups with the following formula: EQU MD.sup.vi.sub.q M PA0 (d) from about 0.0001 parts by weight to about 30 parts by weight of a diluent gum having the following formula: EQU MD.sub.w M PA0 (1) an alkenyl silicone gum or mixture of alkenyl silicone gums selected from the group of alkenyl silicone gums having the formula: EQU (M.sub.a M.sup.vi.sub.1-a)(D.sup.vi).sub.x (D).sub.y (M.sub.a M.sup.vi.sub.1-a) PA0 (2) a vinyl specific peroxide curing agent. PA0 (1) M.sup.vi D.sup.vi DM.sup.vi ; PA0 (2) M.sup.vi DM.sup.vi ; and PA0 (3) MD.sup.vi M.
The consistency of uncured rubber mixtures ranges from a tough putty to a hard deformable plastic. Those rubbers containing reinforcing fillers tend to stiffen on storage due to the development of structure in the filler. Low viscosity fluids added to the rubber, such as water, diphenylsilanediol, or silicone fluids inhibit stiffening and the development of structure.
The properties of fabricated rubber depend not only on the chemical nature of the gum but also on the properties of the filler, additives, and type of curing catalyst. Consequently, the resultant property profile of a given heat cured silicone rubber is highly dependent on the chemical nature of the various constituent components as well as the relative proportions of those components. For example, a high filler content increases hardness and solvent resistance of the resulting rubber. Such increased hardness and solvent resistance however, comes at the price of a reduced elongation.
Not only do the properties of heat cured silicone rubber vary with the nature of the silicone gum and the various additives as well as their respective proportions but the properties also vary as a result of the various procedures used to compound the rubber. Properties of a heat cured rubber may therefore vary as a function of the thoroughness of the mixing and the degree of wetting of the filler by the gum. All other factors being equal, a hydrophilic filler as opposed to a hydrophobic filler will impart significantly different properties to a finished rubber.
Further, properties of heat cured rubbers change with time. This is particularly true during the initial periods of the curing reaction. Since silicone rubbers are complex chemical mixtures, the cure reactions and associated side reactions never completely stop although they may slow down considerably after the initial cure. The properties of a heat cured rubber change slowly with age.
Silicone rubbers may be cured by one of three general curing techniques:
For a hydrosilylation cure, high molecular weight polymers, i.e. gums, possessing a vinyl functionality are reacted with low molecular weight hydride-functional cross-linking agents. A stable platinum complex, functioning as a catalyst, is added along with an inhibitor to prevent cure initiation prior to heating.
Free radical curing of silicone rubbers is effected by heating the rubber precursor in the presence of a free radical initiator such as benzoyl peroxide. The predominant mechanism operating involves hydrogen abstraction from the methyl groups of the dimethylsiloxane moiety followed by radical attack on another methyl group creating a crosslinking ethylene bridge. If a small percentage of vinyl groups are present, the methyl radical can add to the vinylic double bond. In addition to benzoyl peroxide, other radical cure initiators include bis(2,4-dichlorobenzoyl)peroxide, tert-butyl peroxybenzoate, dicumyl peroxide, 2,5-dimethyl-di-(tert-butylperoxy)hexane, and 1,1-di-(tert-butylperoxy)trimethylcyclohexane. Both 2,5-dimethyl-di-(tert-butylperoxy)hexane, and 1,1-di-(tert-butylperoxy)-trimethylcyclohexane are particularly useful and specific as free radical cure initiators for vinyl silicone heat cured rubbers.
High energy radiation, either as gamma rays or as an electron beam, can also effect cures. This type of cure causes a wide variety of bonds to be broken, thus cross-links occur between a variety of different atomic centers as the radicals created by the high energy recombine to form new chemical bonds.
When a heat cured rubber formulation is used to manufacture products such as gaskets, the particular end use and the environment of that end use govern how the material is formulated and processed. In the case of gaskets, compression set, sealing force, and retention of sealing force are important measure s of performance. Compression set has been a significant factor in heat cured rubber technology for many years.
U.S. Pat. No. 2,803,619 discloses a polydimethylsiloxane gum filled with fumed silica and diatomaceous earth having a low compression set. The heat cured rubber of the '619 patent was cured by a peroxide initiated vulcanization lasting five minutes at 150.degree. C. followed by a twenty-four hour cure at 250.degree. C. Subsequently after an additional twenty-two hours at 150.degree. C., the compression set of the finished rubber was measured.
Curing of a heat cured rubber begins when the cure is initiated during the molding process. The cure must be sufficiently rapid that the article can be removed from the mold without deformation. Yet the requirement that the finished product possess elastomeric properties in some degree means that the cure cannot proceed to the extent that the initially elastomeric heat cured rubber is no longer deformable. Thus the kinetics of the cure reaction must be carefully balanced for a rapid initial cure.
Subsequent developments have focused on three technical issues:
In-situ filler treatment may be divided into two broad classes: 1) vinyl silazane treatment of the filler, and 2) vinyl alkoxy silane treatments.
In the case of free-radical cures, generally peroxide initiated, the initiator is consumed. Use of gamma radiation or high energy electron beams also leaves no reactive residues in the rubber. When a hydrosilylation catalyst is used to effect a cure in a vinyl-hydride compound rubber, the cure must be controlled because the catalyst is not destroyed by the cure reaction. Thus a large variety of inhibitor compounds have been used: alkaline earth metal silicates (U.S. Pat. No. 3,817,910), metal sulfides (U.S. Pat. No. 5,219,922), boron compounds (U.S. Pat. No. 4,690,967), and various organic compounds (U.S. Pat. No. 5,153,244).
Additives to heat cured rubbers to control compression set most frequently involve the addition of substituted silicone resins. Recently, in sharp contrast, spinels have been used to control compression set (U.S. Pat. No. 5,260,364). Since the silicone resins added to the heat cured rubber formulation for compression set control are highly branched silicone resins, depending on when these resins are added can sometimes lead to the conclusion that these materials form part of the elastomeric matrix of the heat cured rubber.
A current problem not yet solved by the art deals with the incompletely reacted surface silanol groups of the various silica fillers currently in use. The presence of reactive, i.e. unreacted, surface hydroxyl or silanol groups in a silica filler leads to condensation reactions and structuring of the filler. One solution currently in use is to use silanol or methoxy stopped silicone fluids as blending agents to assist in dispersing the filler into the gum and also provide a reaction center that does not lead to structuring of the filler. In a sense, these blending agents are reactive diluents as they react with the filler surface hydroxyl or silanol groups preventing the condensation reactions between filler particles or filler and gum molecules that lead to stiffening and a loss of elastomeric properties.